Process for the surface treatment of titanium and its alloys

ABSTRACT

METHOD FOR THE SURFACE TREATMENT OF TITANIUM AND ITS ALLOYS WHICH RESULTS IN IMPROVED JOINTS SUBSEQUENTLY FORMED BETWEEN THE METAL AND ORGANIC MATERIALS AND COMPRISES TREATMENT IN AN AQUEOUS SOLUTION CONTAINING HYDROFLUROIC ACID, AT LEAST 60 PERCENT BY VOLUME OF CONCENTREATED PHOSPHORIC ACID, AND NITRIC OR CHROMIC ACID.

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United States Patent 3,676,223 PROCESS FOR THE SURFACE TREATMENT OF TITANIUM AND ITS ALLOYS Hargovind Nihchaldas Vazirani, Passaic Township,

Morris County, N.J., assignor to Bell Telephone Laboratories, Incorporated, Murray Hill, NJ.

Filed Apr. 17, 1970, Ser. No. 29,511 Int. Cl. C23f 7/14 US. Cl. 148-615 R Claims ABSTRACT OF THE DISCLOSURE Method for the surface treatment of titanium and its alloys which results in improved joints subsequently formed between the metal and organic materials and comprises treatment in an aqueous solution containing hydrofluoric acid, at least 60 percent by volume of concentrated phosphoric acid, and nitric or chromic acid.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to the surface treatment of titanium and its alloys by the use of a solution containing hydrofluoric acid, phosphoric acid, and nitric of chromic acid, in order to improve joints subsequently formed between such surfaces and organic materials, and also relates to the joined product.

(2) Description of the prior art Joining titanium and its alloys to organic materials is desirable in many cases as, for example, in the joining of structural titanium parts by means of adhesives. Corrosion products ordinarily present on the surface of the metal interfere with such joining and have in the past generally been removed by degreasing or by treatment with a mixture of hydrofluoric and nitric acids. However, joints subsequently formed on surfaces so treated often degrade with time, resulting in eventual joint failures. Depending upon the temperature, stress, and humidity levels, such failures may occur after only a few days.

SUMMARY OF THE INVENTION This invention is essentially a joining technique in which the surfaces of titanium and its alloys are treated with an aqueous solution containing hydrofluoric acid, at least 60 percent by volume of concentrated phosphoric acid, and an oxidizing acid such as nitric acid or chromic acid. Such treatment results in removal of corrosion products and their replacement by a mechanically strong protective layer, thus enabling formation of joints which exhibit good initial strengths and which resist degradation with time.

Since the surface to be treated ordinarily contains contaminants such as dirt and grease, it is sometimes preferred to remove these prior to the surface treatment and to aid the practitioner, exemplary procedures are briefly described.

Furthermore, the invention includes embodiments in which the treated surfaces of titanium and its alloys may be coated with an organic material, or may be joined by means of an organic material to similar or dissimilar metals and nonmetals.

BRIEF DESCRIPTION OF THE DRAWING The figure is a cross-sectional view of a laminate formed according to the inventive process.

DETAILED DESCRIPTION This process applies to titanium and any of its alloys containing at least 80 percent by weight titanium and has 3,676,223 Patented July 11, 1972 as its principal object the removal of dirt and corrosion products from the metal surface and replacement with a chemically produced layer. It will sometimes be found advantageous to preliminarily clean the metal surface prior to treatment, in order to reduce the rate of contamination of the inventive solution, replacement of which may be both costly and time-consuming. Such preliminary cleaning is by methods well known in the art and thus the following description of preliminary cleaning is intended to be exemplary and not limiting. While preliminary cleaning by degreasing is generally elfective in removing only oils and grease and is ineffective in removing corrosion products such as naturally formed oxide scale, nevertheless, degreasing is in general adequate precleaning for purposes of the invention. D'egreasing may be accomplished by the use of organic solvents such as alcohols, ketones, and chlorinated solvents such as trichloroethylene and perchlorethylene. A more complete description of precleaning methods may be found in Protective Coatings for Metals, third edition, American Chemical Society, Monograph 163 by R. M. Burns and W. W. Bradley, pages 27 to 54, Reinhold (1967).

After preliminary cleaning, or if preliminary cleaning is omitted, before joining, the metal surface must be contacted with the inventive treating solution containing concentrated hydrofluoric acid (containing about 71 percent by Weight of acid), concentrated phosphoric acid (containing about percent by weight of acid) and an oxidizing acid. The oxidizing acid may be concentrated nitric acid (concentrated nitric acid contains about 42 percent by weight acid) or chromic acid. For purposes of this description, chromic acid is defined as a 5 percent by weight solution of solid CrO or its equivalent. As is known, chromic acid may be formed by adding an acid salt or basic salt to an acidic solution. Such salts include bichromate (Cr O-;=) salts and chromate (Cr0 salts, e.g., sodium or potassium bichromate or sodium or potassium chromate. The concentrations of hydrofluoric acid and the oxidizing acid in the treating solution is not critical and may be in the amount of from 0.1 to 39.9 percent by volume, below which frequent replenishment of the treating solution may be required. It is preferred for this reason that the acids be present in the amount of at least 5 percent by weight. Above 20 percent by weight, little is gained in improving the character of the joint finally obtained. Chromic acid is in general preferred over nitric acid for the obtaining of optimum performance of subsequently formed joints.

In addition to the hydrofluoric acid and the oxidizmg acid, it is essential for the practice of the invention that the solution additionally contain concentrated phosphoric acid in the amount of at least 60 percent by volume. Although not completely understood, it is thought that the presence of phosphoric acid controls the rate of attack of the metal surface, thus permitting the formation of a mechanically strong oxide layer thereon. Below 60 percent by volume the amount of concentrated phosphoric acid is insufficient to substantially contribute to the obtaining of a suitable joining surface. For best results, it is preferred that concentrated phosphoric acid be present in the amount of at least 80 percent by volume.

The time and temperature of the treatment are not critical. In general, treatment for at least ten seconds at room temperature (-68" F.) will ordinarily result in a noticeable improvement in the subsequently formed joints. However, treatment times in excess of fifteen minutes and temperatures in excess of F. should in general be avoided, due to the substantial amount of metal which will be removed as a result of such treatment conditions.

The following examples compare adhesive joint strengths obtained by use of the inventive treatment and by prior art treatments.

EXAMPLE 1 Two sets of T-peel joints were made from specimens of titanium having dimensions of about one inch in width and two mils in thickness joined by a nylon epoxy film adhesive, according to a standard testing procedure. Treatment prior to joining was as follows:

Set No. 1 was vapor degreased and set No. 2 was treated with a solution containing by volume 80 percent concentrated phosphoric acid, percent concentrated hydrofluoric acid and 10 percent of an aqueous 10 percent by weight solution of sodium bichromate. The specimens were then rinsed with distilled water and dried. Results were shown in Table I as the average T-peel strength in inch pounds per inch width for each set of specimens.

The procedure of Example 1 was followed except that the thickness of the specimens was 20 mils. The results are shown in Table II.

TABLE II T-peel strength Set No.: I (lb. per inch width) 1 12.1 2 31.6

EXAMPLE 3 Two sets of standard lap joints were made from specimens of titanium approximately one inch wide by 63 mils thick with an epoxy adhesive. Treatment prior to joining was as follows:

Set No. 1 was treated in a solution containing in parts by volume one part hydrofluoric acid and ten parts nitric acid, and set No. 2 was treated with a solution containing by volume 80 percennt concentrated phosphoric acid, 10 percent concentrated hydrofluoric acid, and 10 percent of a 10 percent solution of sodium bichromate. Results are shown in Table HI as the initial joint strength in pounds per square inch and as time to failure under conditions of 95 F. and 90 percent relative humidity, under the static stress shown.

TABLE III Set No. 1 Set No. 2

Initial joint strength (p.s.i.) 4, 790 5, 770 Days to failure (4,500 psi.) 1 4 Days to failure (3,000 p.s.i.) 7 43 ess. Shown in the figure is a titanium or titanium alloy body 1 joined to an organic body 2. Such organic materials include adhesives, coatings, and potting compounds. Examples of adhesive materials are epoxies and modified epoxies, nitrile rubber phenolics, polyvinyl butyrals and polyvinyl formals. Examples of coating materials are alkyds and modified alkyds, acrylics, polyesters, and urethanes. Potting compounds include polyurethanes, silicones and epoxies.

These materials can be applied in the conventional manner. For example, solid, fusible organic materials may be applied to the titanium-containing surface in the molten state and allowed to solidify. Solid, soluble organic materials may be applied in a volatile solvent and the solvent evaporated. Liquid organic materials capable of being cured to a nonfluid state may be applied in their liquid state, with or without added cross-linking agents, and exposed to cross-linking conditions. Solid organic materials such as partially cured films may be applied to the surface and substantially cured in situ, for example by the application of heat and pressure. In each instance the joint is improved by the described treatment of the titanium-containing surface.

What is claimed is:

1. A method for joining a surface containing at least 80 percent by weight titanium to an organic material in which said joining is preceded by the step of contacting said surface with an aqueous solution containing from 0.1 percent to 39.9 percent by volume of hydrofluoric acid, from 0.1 percent to 39.9 percent by volume of an oxidizing acid selected from the group consisting of nitric acid and chromic acid, and characterized in that said solution additionally contains at least percent by volume of phosphoric acid.

2. The method of claim 1 in which said solution contains hydrofluoric acid and the oxidizing acid in the amount of from 5 to 20 percent by volume.

3. The method of claim 1 in which said solution contains phosphoric acid in the amount of at least percent.

4. The method of claim 1 in which contacting said sur face with said solution is carried out for a time of at least 10 seconds.

5. The method of claim 1 in which the step of contacting said surface with said solution is preceded by degreasing.

References Cited UNITED STATES PATENTS 3,010,854 11/1961 Satterfield 252-793 X 3,514,407 5/1970 Missel 134-41 X 3,562,013 2/1971 Mickelson et al 134-3 RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R. 148-6.16 

